1. Technical Field
The present invention relates, generally, to semiconductor packaging and, more particularly, to a press-fit rectifier package employing an improved cup design.
2. Background Information
The need for high-volume assembly of components employing power semiconductor devices, along with the continued need for optimum heat-transfer capabilities for such devices, has led to the development of "press-fit" packages. These packages, most often used for high power rectifier applications, have enjoyed increasing popularity--particularly in the automotive industry, where power rectifiers are often used in multiples of 6 or 8 in alternator designs and the like.
Examples of press-fit and comparable package designs can be found in U.S. Pat. No. 5,005,069, issued Apr. 2, 1991 to Wasmer et al. Referring to the prior art package shown in FIG. 1, a typical prior art package includes a nailhead 104 and cup 114 having a semiconductor die 111 bonded therebetween. The perimeter of die 111 is typically coated with a passivant 110. A mold lock 106 is provided along the bottom of the cup, and a cavity formed by wall 108 is filled with an encapsulant 107.
As illustrated, the press-fit package 102 is inserted into an opening formed within a plate or other form 104 via an applied axial force 120. A compression fit between the inner surface of opening and the outer surface of the cup 114 is formed. As a result, no solder or other bonding agent is needed to provide electrical and mechanical contact between the press-fit package 102 and plate 104.
Currently known press-fit packages such as this are, however, inadequate in a number of respects. For example, known packages are commonly susceptible to die cracking during the press-in operation. That is, die 111 is not sufficiently decoupled mechanically from the package body. Thus, the applied radial compressive force (112) results in high localized tensile stress at one or more locations (typically the comers) of die 111. This can lead to catastrophic die fracture or, in some instances, small scale cracking which propagates during subsequent operation.
Known press-fit packages are also inadequate in that they tend to become misaligned during the press-in operation. More specifically, as it is common for the packages to be dropped into place prior to application of the press-in force, it is possible for the packages to be tilted at an angle, thereby increasing the applied stress experienced by the package during press-in and resulting in a non-optimal contact between the package and the plate. This inadequacy is a function of a number of attributes of the package, including, for example, weight, center of gravity, and exterior profile of the package.
In addition, known packages are often susceptible to thermo-mechanical fatigue during field operation. That is, small-scale contractions and expansions arising during heating and cooling cycles gives rise to high-cycle fatigue of the solder bonds used to attach the die to the nailhead and cup.
Furthermore, many packages, in attempt to address one or more of the aforementioned problems, employ plastic enclosures that are not amenable to high-temperature manufacturing and/or high temperature field operation.
Methods are therefore needed in order to overcome these and other limitations of the prior art. More particularly, there is a long-felt need for robust press-fit packages which eliminate press-in related die cracking, reduce misalignment, reduce failure due to fatigue, and can withstand high-temperature operation.